MarkDH102
Posts: 279
Joined: Fri Feb 13, 2015 3:18 pm

[SOLVED] Question about some VERY old LED displays

Wed Nov 22, 2017 4:07 pm

They are 1 inch high and marked RS 587 377. Common cathode.

A CC (2)
F B
CC (1) N/C
N/C G
N/C C
N/C* DP
E D

* The only similar data sheet I could find said this was Left Decimal Point, but my displays only have a right DP.
I have measured and see that CC1 & CC2 are joined inside the display.
SO, I have wired 4 of them up on some veroboard and connected them to a Pi3 and written some Python to multiplex them.
I've got 22 OHM resistors in line with all the segments and the DP (I only use the DP on one display)
Everything works fine. EXCEPT...when I drive the DP pin on that one display the rest of the segments associated with THAT display go blank, the other 3 displays carry on working fine.
I've checked the wiring for shorts and continuity and everything is fine. If I don't drive the DP high, then the rest of the display works fine.

A long shot, but any ideas?
Last edited by MarkDH102 on Thu Nov 23, 2017 10:16 am, edited 1 time in total.

User avatar
FTrevorGowen
Forum Moderator
Forum Moderator
Posts: 4625
Joined: Mon Mar 04, 2013 6:12 pm
Location: Bristol, U.K.
Contact: Website

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 5:19 pm

Are you driving the display from the Pi's GPIO's (via the 22 ohms etc.) What colour are they? (Older red leds required higher currents that present devices).
FWIW, this is what I used for an old, small RS 4-digit display:
http://www.cpmspectrepi.uk/raspberry_pi ... oFull.html
Trev.
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

jdb
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 1791
Joined: Thu Jul 11, 2013 2:37 pm

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 5:49 pm

Do you have a multimeter with diode tester functionality?

Testing N pins to every other pin in turn (for NxN tests, both polarity - or fewer once you find the common anode or cathode) would tell you the connection and orientation of each diode. The LED will also glow faintly in the forward-biased direction.
Rockets are loud.
https://astro-pi.org

hippy
Posts: 4014
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 6:39 pm

And don't forget to double-check the code to make sure you are not doing something silly like clearing all the other segment bits when the DP is set.

You could possibly swap the DP GPIO with some other segment GPIO to see if it exhibits similar behaviour.

MarkDH102
Posts: 279
Joined: Fri Feb 13, 2015 3:18 pm

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 7:08 pm

Thanks for all your suggestions.
The displays are red.
I drove individual segments on a breadboard to start with for testing purposes using the 3.3v supply from an Arduino Uno via a 22 ohm resistor and measured 3.2mA per segment.
I use the code on other projects so I will set to checking my wiring...

User avatar
FTrevorGowen
Forum Moderator
Forum Moderator
Posts: 4625
Joined: Mon Mar 04, 2013 6:12 pm
Location: Bristol, U.K.
Contact: Website

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 8:02 pm

MarkDH102 wrote: Thanks for all your suggestions.
The displays are red.
I drove individual segments on a breadboard to start with for testing purposes using the 3.3v supply from an Arduino Uno via a 22 ohm resistor and measured 3.2mA per segment.
I use the code on other projects so I will set to checking my wiring...
IIRC, 3.2mA is somewhat high for a pi's GPIO to provide - for single LEDs a series resistor of ~220 - 560 ohms (depending on LED colour) is more common (associated with a current of ~1mA). Hence my use of 3.3V to 5.0V level shifters and 5V I.C.'s capable of providing greater (or controlled, limited) currents. FWIW, there are other example LED display circuits within my webpages (as well as the "early" one I linked to).
Trev.
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

drgeoff
Posts: 8744
Joined: Wed Jan 25, 2012 6:39 pm

Re: Question about some VERY old LED displays

Wed Nov 22, 2017 10:04 pm

Individual RPi GPIOs are reckoned to be good for up to 16 (sixteen) mA each. There is also a supposed maximum of a total of 50 (fifty) mA for all GPIOs but I don't recall seeing an authoritative answer of whether this is a chip limitation which still applies or was all that was available from the linear 3.3 volt regulator on earlier model RPis once the on-board stuff took what they need.

User avatar
jojopi
Posts: 3042
Joined: Tue Oct 11, 2011 8:38 pm

Re: Question about some VERY old LED displays

Thu Nov 23, 2017 2:04 am

drgeoff wrote:
Wed Nov 22, 2017 10:04 pm
There is also a supposed maximum of a total of 50 (fifty) mA for all GPIOs but I don't recall seeing an authoritative answer of whether this is a chip limitation which still applies or was all that was available from the linear 3.3 volt regulator on earlier model RPis once the on-board stuff took what they need.
It is a chip or package limitation. The 50mA-per-bank limit is included in the Compute Module datasheet, where you provide your own power supplies.

(On consumer Pies, only bank 0 is accessible. I am still not clear how the limit should apply to combinations of GPIOs sinking and sourcing current.)

MarkDH102
Posts: 279
Joined: Fri Feb 13, 2015 3:18 pm

Re: Question about some VERY old LED displays

Thu Nov 23, 2017 10:15 am

Ok, I've bread-boarded a spare display and if I put a 470 OHM (220 didn't work) in series with the DP and leave the 22 OHM on the segments, it works (DP quite dim).
I don't think the displays are suitable for being driven from 3.3V on the Pi.
I also used the 5V of the Arduino UNO to power the segments with exactly the same problem with the DP.
So, I have no idea what voltage spec these displays are. They are at least 35 years old!
It was worth a try...

hippy
Posts: 4014
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: [SOLVED] Question about some VERY old LED displays

Thu Nov 23, 2017 12:09 pm

That you can light the DP and all segments, even if not together, shows the voltage is good enough to drive those. The only thing I can think is that the segments require more voltage than the DP does; turning the DP on with other segments increases GPIO current, causes the GPIO voltage to drop below what will turn the segments on.

You measured the current and that was 3.2mA per segment at 3.3V with 22R. That suggests a Vfwd of around 2.6V per segment. You would only need a 0.7V voltage drop to have those stop working.

If the segments have two LEDs, the DP only one, with a 1.3 Vfwd a 22R would mean 90mA. That could cause such a voltage drop, is well in excess of what each pin of GPIO can take and what the GPIO can take in total.

I would say try a 680R for the DP but you seem to suggest that's not bright enough.

User avatar
davidcoton
Posts: 3252
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: [SOLVED] Question about some VERY old LED displays

Thu Nov 23, 2017 8:42 pm

Following hippy's thought: if you did try the DP at 3V3 with 22R, you may have damaged that LED resulting in reduced brightness. Do you have a digit with an untried DP to check with 680R?
"Thanks for saving my life." See https://www.raspberrypi.org/forums/viewtopic.php?p=1327656#p1327656
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”

MarkDH102
Posts: 279
Joined: Fri Feb 13, 2015 3:18 pm

Re: [SOLVED] Question about some VERY old LED displays

Fri Nov 24, 2017 4:23 pm

There are indeed 2 LED's in each segment which explains a voltage drop causing them not to light.
I breadboarded a spare device and dropped a 470 OHM in series with the DP and it lights up at the same intensity as the one on my veroboard (now also using a 470 OHM).
The segments on all the displays are really quite dim so I will either look at driving them "properly" using an external driver or I will just dig out my .56" displays and use them instead.
Like I say, they are really old so I have no idea what their real specifications are.

Return to “Off topic discussion”